A 17 year old male patient presented with short stature and delayed puberty. Investigations showed normal thyroid function and intact TSH response to TSH-releasing hormone (TRH). Although basal levels of LH were low, both LH and FSH rose following the administration of LH-releasing hormone (LHRH). ACTH secretion assessed indirectly by the cortisol response to insulin hypoglycemia was normal. Growth hormone levels increased following the onset of sleep, as well as after the administration of insulin, L-dopa and L-arginine. Basal levels of prolactin were low (2-5 ng/ml) compared with 5-12 ng/ml in controls. There was a markedly impaired prolactin response to TRH (maximum rise above basal values of 3 ng/ml compared to a rise of 12-29 ng/ml in controls). Prolactin levels did not rise after the administration of chlorpromazine or L-arginine. There was some suppression of prolactin levels after L-dopa. Similar patterns of prolactin are seen in panhypopituitarism, where they are usually associated with other hypophyseal hormonal deficiencies. The diminished prolactin reserve demonstrated in this subject in the presence of intact function of the remainder of the anterior pituitary is compatible with the diagnosis of diminished prolactin reserve.